Semimetallic Two-Dimensional Boron Allotrope with Massless Dirac Fermions (1309.2596v2)

Abstract: It has been widely accepted that planar boron structures, composed of triangular and hexagonal motifs are the most stable two dimensional (2D) phases and likely precursors for boron nanostructures. Here we predict, based on ab initio evolutionary structure search, novel 2D boron structure with non-zero thickness, which is considerably, by 50 meV/atom lower in energy than the recently proposed alpha-sheet structure and its analogues. In particular, this phase is identified for the first time to have a distorted Dirac cone, after graphene and silicene the third elemental material with massless Dirac fermions. The buckling and coupling between the two sublattices not only enhance the energetic stability, but also are the key factors for the emergence of the distorted Dirac cone.

• The paper introduces a novel 2D boron allotrope with a distorted Dirac cone that supports massless Dirac fermions.
• It employs ab initio evolutionary structure searches with both GGA and HSE06 methods, ensuring robust lattice stability analysis.
• Findings suggest the allotrope has lower energy than traditional α-sheets and paves the way for advanced electronic applications.

Overview of "Semimetallic Two-Dimensional Boron Allotrope with Massless Dirac Fermions"

The paper authored by Xiang-Feng Zhou et al. reports on an innovative finding within the arena of 2D materials, specifically focusing on a novel two-dimensional (2D) boron structure. This investigation leverages ab initio evolutionary structure search methodologies and identifies boron-based structures with compelling and significant electronic properties. Notably, these structures exhibit the presence of massless Dirac fermions, which represent a property of high interest given the potential applications in electronic and quantum devices.

Computational Approach and Findings

The research utilized the ab initio evolutionary algorithm USPEX to explore potential 2D boron polymorphs, capitalizing on its successful application to bulk materials predictions in prior studies. These calculations involved rigorous relaxation processes to determine the most stable structures. Energies were computed using both the generalized gradient approximation (GGA) and the hybrid HSE06 functional to ensure accurate lattice parameters and stability conclusions.

Two orthorhombic structures emerged as prominently stable, denoted as $Pmmn$-boron and $Pmmm$-boron. The $Pmmn$-boron phase is identified as an inherently stable 2D configuration lower in energy than the previously known $\alpha$-sheet, with energy approximately 50 meV/atom less than the $\alpha$-sheet structure. The key to its stability lies in its non-zero thickness—features buckling and bilayer coupling that facilitate stability—and its intriguing electronic properties, particularly the emergence of a distorted Dirac cone in its electronic band structure.

Electronic Properties

The $Pmmn$-boron exhibits a zero-gap semiconductor nature, akin to graphene, and harbors a distorted Dirac cone where the Dirac point manifests without necessitating a hexagonal symmetry. The anisotropic Dirac cone unveiled in $Pmmn$-boron is a rare and notable characteristic within non-carbon-based materials, positing it as a distinct counterpart to graphene and silicene. The implications are profound: the discovery expands the understanding of Dirac fermions to orthorhombic symmetry systems beyond graphene's typical systemic environment, with band slopes indicating direction-dependent electronic properties.

Charge density analysis at the Dirac point further supports the origin of these properties, showcasing hybrids of in-plane and out-of-plane states across the orthorhombic lattice, correlating to the distinctive buckled atomic configurations that form the Dirac cone.

Implications and Future Directions

The identification of these orthorhombic boron allotropes offers foundational insights into the synthesis and applications of new materials exhibiting Dirac fermion behavior. From a practical standpoint, acknowledging the likelihood of synthesizing these structures on metal substrates propels forward the possibility of experimental realization. The characteristic properties of $Pmmn$-boron suggest promising avenues for electronic devices leveraging direction-dependent electronic behavior.

Theoretically, the paper's findings could stimulate a reevaluation of synthetic strategies for boron sheets and nanotubes, incorporating finite thickness considerations. This could facilitate the design of novel nanostructures that leverage the dynamics of Dirac electron propagation in non-planar configurations.

Overall, this research enriches the understanding of boron's versatility in 2D allotropes, paving the way for potential breakthroughs across electronic, optoelectronic, and quantum computing domains. Moving forward, experimental validation and exploration of substrate effects are suggested, alongside theoretical investigations that deepen the understanding of the stability and electronic properties mechanism at play in these boron structures.